The initial stages of solid solution decomposition with formation of Guinier-Preston zones are theoretically analyzed. It is concluded that the volume diffusion at this stage is not sufficiently advanced because of inconsiderable concentration gradient. The motive force of precipitation is a force of interatomic interaction between the nearest neighboring atoms. The formation of interfaces with lower chemical potential in comparison with that of surrounding solid solution initiates the volume diffusion of atoms in the direction of initial precipitates. So, the extrapolation of diffusion coefficient values obtained from high temperature experiments to the stage of spinodal precipitation is shown to be not correct.